Very good remark Jim,
Another complicating factor is that for intel and amd there is
definitely ready available libraries written in assembly that are
lightning fast,
whereas for C* type processors i can't remember there is much, as
your "more Ghz into battle is better rule" always means that the C*
chip is gonna be slower anyway,
also in future incarnations over intel&amd, so why bother writing
good libraries for that chip?
Vincent
On Nov 8, 2007, at 11:43 PM, Jim Lux wrote:
> At 11:04 AM 11/8/2007, Peter St. John wrote:
>> Vincent,
>> That's tough for me to answer, presumably the 1.5 is cheaper per
>> hertz
>> in power than a 3 GHz, but because of the other issues it may not be
>> cheaper in GFLOPS per power. No hablo EE.
>> Peter
>>>> On Nov 8, 2007 1:58 PM, Vincent Diepeveen <diep at xs4all.nl> wrote:
>> > For a compute cluster wouldn't it be a thought to also consider the
>> > cost of 3 years of nonstop electricity for the amount of gflops it
>> > delivers?
>> >
>>> In general, a N GHz processor will be poorer in a flops/Watt sense
> than a 2N GHz processor.
>> The power draw is a combination of a fixed load plus a frequency
> dependent load, so for the SAME processor, running it at N/2 GHz
> consumes more than 50% of the power of running it at N GHz.
>> If you go to a faster processor design, the frequency dependent
> load gets smaller (smaller feature sizes= smaller capacitance to
> charge and discharge on each transition). The core voltage is also
> usually smaller on higher speed processors, which also reduces the
> power dissipation (smaller number of joules to change the voltage
> from zero to one or vice versa). So, in general, a 2N GHz
> processor consumes less than twice the power of a N GHz processor.
>> Complicating this all is:
> a) A significant fraction of the load in a PC is all the other
> stuff that's toggling back and forth, like memory address and data
> lines. This will be driven more by the FSB speed, which might be
> the same for the two processors.
> b) You may have a lower core voltage, but the regulator making that
> voltage may or may not be as efficient.
> c) Power supply efficiency can vary a LOT from model to model. All
> the way from 50% for a really crummy design to 95% for a good design.
> d) Faster processors aren't necessarily architecturally identical
> to slower processors. They may have different pipeline depths,
> different microcode, different ALU strategies, etc. It's not just
> a matter of shrinking the masks and turning up the clock.
>>> Jim
>>>